Magnetic Random Access Memory (MRAM) devices are nonvolatile memory devices which can be operated at relatively low voltages and at relatively high speeds. In a unit memory cell of an MRAM device, data is stored in a magnetic tunnel junction (MTJ) structure of a magnetic resistor. The MTJ structure may include first and second ferromagnetic layers and a tunneling insulation layer therebetween. A magnetic polarization of the first ferromagnetic layer (referred to as a free layer) may be changed using an external magnetic field that crosses the MTJ structure. The external magnetic field may be induced by a current that flows adjacent the MTJ structure, and a magnetic polarization of the free layer may be parallel or anti-parallel with respect to the fixed magnetic polarization of the second ferromagnetic layer (referred to as a pinned layer). Current used to create the external magnetic field may flow through conductive layers such as a digit line and a bit line, disposed adjacent the MTJ structure.
According to spintronics based on quantum mechanics, when magnetic spins in the free layer and the pinned layer are parallel with respect to each other, a tunneling current passing through the MTJ structure may have a relatively high value (i.e., the resistance of the MTJ structure may have a relatively low value). When the magnetic spins in the free layer and the pinned layer are anti-parallel with respect to each other, a tunneling current passing through the MTJ structure may have a relatively low value (i.e., the resistance of the MTJ structure may have a relatively high value). Accordingly, data of an MRAM cell can be determined according to a direction of the magnetic spins in the free layer.
Most MTJ structures have a rectangular shape or an elliptical shape when viewed from direction perpendicular to the substrate. These shapes may be provided because the magnetic spins in the free layer may have a relatively stable state when the magnetic spins in the free layer are parallel to a longitudinal direction of the free layer.
An MRAM device may include a plurality of MTJ structures, and MTJ structures may exhibit non-uniform switching characteristics according to fabrication operations used. External magnetic fields used to store desired data in different MTJ structures may differ. Accordingly, increased non-uniformity of switching characteristics of MTJ structures may result in reduced writing margins for the MRAM device. In particular, when MTJ structures are scaled down for increased integration density, writing margins may also be reduced. In other words, during a writing operation to selectively store a desired data bit in one of the MTJ structures, undesired data may be written in non-selected MTJ structures that share a bit line and/or a digit line electrically connected to the selected MTJ structure. According to conventional writing methods, a write disturbance may thus occur during an operation used to store data in the selected MTJ structure.
Furthermore, a conventional MRAM cell may include a digit line disposed adjacent the MTJ structure as discussed above. In general, the digit line is provided between the MTJ structure and the substrate, and the MTJ structure has a bottom electrode overlapping the digit line. In this case, the bottom electrode may be electrically connected to a drain region of an access transistor provided below the digit line. Thus, the bottom electrode may extend in a horizontal direction to contact a contact plug formed on the drain region. As a result, reductions in a planar area of the MRAM cell may be difficult due to the presence of the digit line.
MRAM devices suitable for application of a spin injection mechanism have been proposed to reduce write disturbance and to increase integration density. For example, MRAM devices suitable for application of a spin injection mechanism are discussed in U.S. Pat. No. 6,130,814 to Sun, entitled “Current-induced magnetic switching device and memory including the same”. In addition, other MRAM devices suitable for application of the spin injection mechanism are disclosed in U.S. Pat. No. 6,603,677 B2 to Redon et al., entitled “Three-layered stacked magnetic spin polarization device with memory”. The disclosures of U.S. Pat. No. 6,130,814 and U.S. Pat. No. 6,603,677 are hereby incorporated herein in their entirety by reference.